The present invention relates generally to communication systems, and more particularly to interference mitigation and noise whitening in such communication systems.
The structure and operation of high-speed data communication systems are generally known. Such high-speed data communication systems employ various media and/or wireless links to support the transmission of high-speed data communications. Particular embodiments of high-speed communication systems include, for example, cable modem systems, home networking systems, wired local area networks, wired wide area networks, wireless local area networks, satellite networks, etc. Each of these high-speed data communication systems has some unique operational characteristics. Further, some of these high-speed data communication systems share similar operational drawbacks. Home networking systems and cable modem systems, for example, are both subject to interfering signals that are coupled on media that carry communication signals.
Cable modem systems, and more generally, cable telecommunication systems include set-top boxes and residential gateways that are in combination capable of currently providing data rates as high as 56 Mbps, and are thus suitable for high-speed file transfer, video teleconferencing and pay-per-view television. These cable telecommunication systems may simultaneously provide high-speed Internet access, digital television (such as pay-per-view) and digital telephony. Such a system is shown and described in U.S. patent application Ser. No. 09/710,238 entitled xe2x80x9cPre-Equalization Technique for Upstream Communication Between Cable Modem and Headendxe2x80x9d, filed on Nov. 9, 2000, the disclosure of which is expressly incorporated by reference.
Cable modems are used in a shared access environment in which subscribers compete for bandwidth that is supported by shared coaxial cables. During normal operations, sufficient bandwidth is available across the shared coaxial cables to service a large number of subscribers, with each subscriber being serviced at a high data rate. Thus, during normal operations, each subscriber is provided a data rate that is sufficient to service uninterrupted video teleconferencing, pay-per-view television, and other high bandwidth services.
Intermittent, narrowband interfering signals may, from time to time, interfere with wideband communication signals, e.g., upstream Data-Over-Cable Service Interface Specification (DOCSIS) transmissions (xe2x80x9cdesired signalsxe2x80x9d). These intermittent narrowband interfering signals unintentionally couple to the shared coaxial cables via deficiencies in shielding and/or other coupling paths. With these interfering signals present, the data rate that is supportable on the coaxial cables is reduced. In some cases, depending upon the strength and band of the interfering signals, the supportable bandwidth is reduced by a significant level.
Conventionally, when an interfering signal is present, an adaptive cancellation filter is employed by each cable modem receiver to cancel the interfering signal by adaptively placing a filtering notch or null at the frequency of the interfering signal. When the interfering signal becomes absent, the conventional adaptive cancellation filter continues to adapt and removes the filtering notch. If the interfering signal reappears, the adaptive cancellation filter again adapts to null the interfering signal. Thus, when the interfering signal first reappears, the cancellation filter cannot fully compensate for the interfering signal. Because many interfering signals are intermittent, the presence of these intermittent signals reduces the bandwidth that is supportable upon the coaxial media during the time period required for the cancellation filter to adapt. Further, because the interfering signals oftentimes vary in strength while present, the adaptive cancellation filter most often times does not fully remove the interfering signal.
Overlapping adjacent channel signals cause another source of interference for the desired signal because they often produce interfering signals in the band of the desired signal. For example, a TDMA signal that resides in an adjacent channel and that turns on and off may have side lobes that overlap and interfere with the desired signal. When the interfering signal is present, the conventional adaptive cancellation filter places a notch or null at the frequency band of the interfering signal. When the interfering signal is absent, the cancellation filter adapts to removes the notch. The precise amount of interference in the desired signal caused by the adjacent channel signals may vary with data content in the adjacent channel.
Thus, in both the case of the narrowband interferer and the adjacent channel interferer, the interfering signal(s) varies over time. For this reason, an optimal or near-optimal solution may be found only for the average interfering strength of the interfering signal(s), but not for the peak(s) of the interfering signal(s). In many operational conditions, typical fluctuations in the strength of interfering signals cause conventional cancellation filters to provide insufficient cancellation. Resultantly, overall bandwidth that could be provided by the supporting communication system on the particular shared media is significantly reduced.
Home phone line networks suffer from similar operational difficulties. Home phone line networks typically employ unshielded twisted pair (UTP) wiring for the communication of data within a premises. UTP wiring is susceptible to RFI coupling because of its construction and RFI sources, such as HAM radio transmitters, couple significant interfering signals onto the UTP wiring. Further, because Plain Old Telephone System (POTS) equipment shares the UTP wiring with the coupled data communication equipment, signals produced by the POTS equipment and introduced by the telephone network introduce additional interfering signals that affect data communications on the home phone line network.
Therefore, there is a need in the art for a filtering system and associated operations that cancel interfering signals and whitens colored noise in a communication system so that throughput is maximized.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art to efficiently whiten colored noise present at the input of a receiver in the communication network. According to the present invention, noise present at the input of the receiver is sampled to produce a sampled signal. Then, the sampled signal is spectrally characterized across a frequency band of interest to produce a spectral characterization of the sampled signal. This spectral characterization does not include the signal of interest but does include colored noise that may include one or more interfering signals. In most embodiments, the signal of interest is a modulated carrier that carries data and that occupies a frequency band. The spectral characterization is then modified to produce a modified spectral characterization. Filter settings are then generated based upon the modified spectral characterization. Finally, the input to the receiver is filtered using the filter settings when the signal of interest is present at the input to whiten co-existent colored noise.
Various operations are employed to modify the spectral characterization to produce the modified spectral characterization. In most cases, spectral components of the spectral characterization are independently modified to produce the modified spectral characterization. Modifications to the spectral characterization are typically performed in the frequency domain but may also be partially or fully performed in the time domain. Examples of spectral component modification include amplification, weighting, and averaging of the spectral components. With tone amplification, each spectral component is amplified by a particular gain factor that is based upon the magnitude of the spectral component and a plurality of thresholds. The gain factor may also be chosen based upon a noise power characterization of the spectral characterization. The spectral components may be weighted based upon the spectral component, prior spectral components, noise power characterization of the spectral characterization, and/or prior noise power characterization(s). The spectral components may be averaged with corresponding spectral components of prior spectral characterizations in determining the modified spectral components/spectral characterization.
In one embodiment, the spectral characterization is folded from its full bandwidth into one-half of its sampled bandwidth. In such operation, pairs of spectral components are added. Filter settings are generated based upon the folded spectral characterization. Because the folded spectral characterization has one-half of the bandwidth of the spectral characterization, the filter settings must be applied to the filter to compensate for the reduced bandwidth. In one operation, the filter settings are applied to alternate taps of the filter, with remaining taps set to zero. Resultantly, the filter whitens colored noise according to the spectral characterization but the computations required to generate the filter settings are significantly reduced.
The filtering operations of the present invention may be deployed within receiver sections of home phone line network devices. When deployed in these devices, the filtering operations remove interfering signals and whiten colored noise present at the input of the receiver. Each of these operations improves the properties of the input and allows data throughput rates to be increased.
Other features and advantages of the present invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawings.